1. Field of the Invention
The present invention generally relates to global position system (GPS) devices and, more particularly, to compensating an oscillator used to receive satellite signals within a location-enabled wireless device.
2. Description of the Related Art
The process of measuring a global positioning system (GPS) signal begins with a procedure to search for the GPS signal in the presence of noise by attempting a series of correlations of the incoming signal against a known pseudo-random noise (PRN) code. The search process can be lengthy, as both the exact frequency of the signal and the time-of-arrival delay are unknown. To find the signal, receivers traditionally conduct a two dimensional search, checking each delay possibility at every possible frequency. To test for the presence of a signal at a particular frequency and delay, the receiver is tuned to the frequency, and the incoming signal is correlated with the known PRN code delayed by an amount corresponding to the time of arrival. If no signal is detected, the search continues to the next delay possibility, and after all delay possibilities are checked, continues to the next frequency possibility. Each individual correlation is performed over one or more milliseconds in order to allow sufficient signal averaging to distinguish the signal from the noise. Such averaging is known as “integration”. Because many thousand frequency and delay possibilities are checked, the overall acquisition process can require tens of seconds.
Recently, new applications of GPS technology in wireless devices have emerged, for example, imbedding GPS receivers in cellular phones to provide emergency location capability. In these applications, rapid signal acquisition in just a few seconds is required. Furthermore, these applications require a GPS receiver to operate in harsh signal environments and indoors, where GPS signal levels are greatly attenuated. Detecting attenuated signals requires each correlation to be performed over a relatively long period of time. For example, integration may be performed over a few seconds, as opposed to the 1-10 millisecond period used in traditional GPS receivers. The two-dimensional sequential search process employed by traditional receivers becomes impractical at such long integration times, because the overall search time increases by a factor of 100 or more.
The performance of a GPS receiver is greatly affected by the frequency stability of the oscillator used to receive the satellite signals. A high stability oscillator source may dramatically decrease the amount of time required to fix location by eliminating the need to search across a wide range of frequency bins. Furthermore, a high stability oscillator source enables the use of long integration intervals in the GPS receiver, greatly enhancing the ability to detect attenuated satellite signals.
In some cases, GPS receivers utilize a temperature compensated crystal oscillator (TCXO) to provide a stable frequency source. TCXOs employ a-priori temperature models or characteristics to adjust output frequency as a function of temperature. Cost effective TCXOs are available that provide stability on the order of two parts per million. While this is adequate for conventional GPS receivers, such stability is not adequate for GPS receivers designed to operate very quickly, with long integration intervals.
U.S. Pat. No. 5,629,708 describes a method of performing temperature compensation of an oscillator using a temperature sensor. The method uses frequency error measurements from GPS signals to adjust a temperature model for a TCXO over time. Adjusting the temperature model over time improves oscillator accuracy when compared to continuous use of an a-priori temperature model. The adjustment process requires the GPS receiver to be operated on a regular basis to keep the temperature model accurate. Such a technique, however, is unsuitable for cellular telephone phone emergency location using GPS, where the GPS receiver may be unused for extended periods. Furthermore, the technique requires that the GPS receiver have the capability of performing Doppler measurements, which typically requires a tracking loop receiver, as opposed to a matched filter receiver often preferred in cellular telephone applications.
Therefore, there exists a need in the art for a method and apparatus that compensates an oscillator in a GPS-capable wireless device without employing frequency error measurements obtained from satellite signals.